JPS6063500A - Rapid neutron therapy variable collimator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable collimator for fast neutron beam therapy that prevents dose reduction by narrowing down the irradiation field and reduces the occurrence of penumbra in fast neutron beam irradiation therapy.

As is well known, fast neutron beams have a large penetrating power through substances, so in order to use them for treatment, it is necessary to thicken the part shielded by a collimator and to make the irradiated part on the irradiation surface have a histogram-like shape. Variable collimation was used.

Figures 1 (al, b) and t (c) are perspective views showing the shape of a conventional parallel plate laminated collimator and the shape of the irradiation part;
They are a sectional view taken along the IT-I line and a sectional view taken along the ■-... line.

In these figures, ] is a fast neutron beam source, a 2-parallel plate laminated collimator, 3 is a flimator plate having a predetermined shielding thickness l, and a predetermined number of collimator plates 3 are laminated. A collimator 2 is formed. 4 is an irradiation space formed by the collimator plate 3 and through which the fast neutron beam passes; 5 is an irradiation surface to which the continuous neutron beam is irradiated; 6 is an irradiation field having a histogram-like shape; 7 is a complete 8 is a shielding part, and 9 is a penumbra part.

In this way, the plate laminated collimator 2 is disposed between the radiation source 1 and the irradiation surface 5, and the fast neutron beam from the radiation source 1 provides an irradiation field 6 having a histogram-like shape.

By the way, in the conventional parallel plate laminated collimator 2, the irradiation surface 5 is completely irradiated with the fast neutron beam from the radiation source 1 passing through the irradiation space 4, as shown in FIGS. 1(b) and 1(c). 7, a shielding part 8 which is shielded by the shielding thickness l of the collimator plate 3, and a penumbra part 9 which is smaller than the shielding thickness l. As can be seen from the figure, this penumbra 9 has the disadvantage that it becomes larger as the shielding thickness l becomes larger and as the width W, W2 of the irradiation space 4 becomes wider.

In order to overcome the above-mentioned drawbacks, a cone-segmented collimator was previously proposed.

Figures 2 (a), (b), and (c) are perspective views showing the shape of a conventional cone-divided frimeter and the shape of the irradiation part;
-A cross-sectional view taken along the I line, and a cross-sectional view taken along the ■-■ gland.
In these figures, the same reference numerals as in FIG. 1 indicate the same parts, 11 is a cone-divided collimator, 12 is a collimator plate, 13 is an irradiation space, 14 is an irradiation field having a histogram-like shape, S is a virtual axis including the radiation source 1.

In this way, as shown in FIG. 2(c), the collimator plate 12 crosses a cross section consisting of an arc of radius n, , R2 centering on the radiation source 1 and a radial line segment dividing the irradiation surface 5 from the radiation source 1.
This is a stack of parts of rings produced by rotating around an imaginary axis S-8 containing . Therefore, the inner surface of the collimator 11 conforms to a polyhedral pyramid having the radiation source as its apex, thereby obtaining an irradiation field 14 having a histogram-like shape as shown in FIG. 2(a). Therefore, the collimator in the case of FIG. 2 can be said to be an ideal flimator because it does not produce the penumbra 9 as in FIG. 1.

However, since the actual radiation source is not a point radiation source but a volume radiation source with a certain spatial extent, the following two drawbacks arise.

First, since it is a volume radiation source, it is impossible to avoid the occurrence of a penumbra. The occurrence of this penumbra is the first
It is smaller than the parallel plate laminated type collimator 2 shown in the figure, so
I have to accept it to some extent. The second drawback is that when the opening degree of the collimator plate 12 is narrowed down, the irradiation dose decreases.

Therefore, when performing precise irradiation treatment, non-uniformity of irradiation weight becomes a major problem leading to uncertainty in treatment. The state in which the irradiation dose decreases will be explained with reference to FIG.

In Fig. 3, 21 is a thin disc-shaped volume m source (hereinafter simply referred to as a radiation source) with a spatial extent and a radius γ.
The same reference numerals as in Figure 2 of the thin tube indicate the same parts. The opening degree of the frimeter 11 with an irradiation width of 2X is an angle θ between the center 0 of the radiation source 21 and points A and B at both ends of the irradiation width 2X.
On the other hand, the range where the m source 21 can be seen from the center O" of the irradiation field 14 is the range BF within the width CD of the source 21, and
Since the fast neutron beams from the CE and DF portions of 1 pass through the collimator plate 12, they are subject to an attenuation effect. The same explanation holds true for any point on the irradiation field 14 other than the center 01.

This invention was made to eliminate the above-mentioned drawbacks, and provides a variable collimator for fast neutron beam therapy that reduces the irradiation dose by narrowing down the irradiation field and keeps the occurrence of penumbra within a range that does not cause any practical problems. It is something to do. This invention will be explained below.

FIG. 4 is an explanatory diagram showing an embodiment of the present invention.
In this figure, 31 is a cone-divided collimator used in the present invention, 32 is a collimator plate, 33 is an irradiation space, and 34 is an irradiation surface.

The collimator plate 32 is made of a laminated collimator plate consisting of a circular arc and a part of a ring generated by rotating a cross section consisting of the radiation emitted from the origin of the radius of this circular arc, and the apex of the cone is located at the irradiation part 34. Point G from the center O1 to the maximum irradiation end (Xmax).

0 and the outermost point C7a of the radiation source 2, and the extension line of the central axis connecting the center 01 of the irradiation surface 34 and the center point 0 of the radiation source 21. It is set at a remote position on the opposite side to the radiation source 21.

As a guide, the maximum irradiation end G, ()]) of the irradiation section 34 and the minimum irradiation end A (B), the points G', (H') of the kebo%, and the radiation source 2
The vicinity of the intersection (1) of the extension of the point C0 of 1 with the extension of the central axes o and o' is defined as the apex of the cone forming the frimeter plate 32.

As described above, assuming that the radiation source 21 has a disk-like shape with a radius γ, the arrangement of the collimator plate 32 with the smallest penumbra on the irradiation surface 34 of a certain size is at the point at the irradiation end, for example. As a result of calculations, it has become clear that G, (F()) almost coincides with the intersection point ■ on the line connecting the outer end C0 of the radiation source 21. This means that if the irradiation end H changes, the collimator The apex ■ of the collimator plate 32 will change each time, but it is impossible to change the geometrical shape of the actual flimator plate 32 each time.For this reason, for example, the apex of the collimator plate 32 can be changed as shown in FIG. When set to position 11, if the irradiation space 33 is large, the penumbra will be small, but if the irradiation space 33 is large, the penumbra will be small.
When the value is made smaller, the penumbra increases and the irradiation space 33 becomes closer to the radiation source 2.
1, the parallel plate laminated collimator 3 shown in FIG. 1 has a smaller penumbra. and,
The reason why the irradiation dose decreases when the irradiation field is narrowed is due to the increased overlap of the penumbra. However, collimator plate 3
By determining the origin 2, that is, the rotation center of the collimator plate 32 at the position 1 as described above, it becomes possible to realize a collimator plate 32 with a performance that does not pose any practical problems. The optimal value for the above position 1 will vary slightly depending on the treatment conditions, but as mentioned above, the maximum irradiation field 0'G (0'H) and the minimum irradiation field (
If it is set based on the G1(H point, which is between 0'B), 1j(l) The penumbra will not become extremely large even if the G1 area expands or narrows. .
Although the above explanation was made regarding a thin disc-shaped volume radiation source, it is easy to understand that the second explanation holds true even if the secondary effects due to the thickness of the radiation source are taken into consideration.

As explained above, the present invention provides an extension line connecting the origin of the cone-split collimator to the maximum irradiation end of the irradiation part irradiated from the volume radiation source and the outermost end of the volume radiation source, and the irradiation part. Since it is set at a position at least on the opposite side of the volume radiation source and away from the intersection of the volume radiation source with the extension of the central axis, the penumbra is small when the collimator is narrowed down, and the irradiation dose is reduced. This method has the advantage that precise irradiation treatment can be performed because an irradiated area with less radiation can be obtained.

[Brief explanation of drawings]

Figures 1 (a), (b), and (c) are perspective views showing the shape of a conventional parallel plate laminated collimator and the shape of the irradiation part;
- A cross-sectional view taken along the I line and a cross-sectional view taken along the nu line. Figures 2 (a), (b), and (c) are perspective views showing the shape of a conventional cone-split collimator and the shape of the irradiation part;
A cross-sectional view taken along the line -■, a cross-sectional view taken along the line IV-IV, FIG. 3 is an explanatory diagram showing the state in which the irradiation dose decreases when the cone-split collimator of FIG. 2 is used, and FIG. 4 is an illustration of the present invention. FIG. 2 is an explanatory diagram showing an example. In the figure, 5 is an irradiation surface, 21 is a volume radiation source, 31 is a collimator, 32 is a collimator plate, 33 is an irradiation space, and 34 is an irradiation surface. 89798 8979B (b) (Figure 3, Figure 4)

Claims (1)

[Claims] Laminated collimator plates consisting of two circular arcs with different radii centered on the origin and a part of a ring created by rotating a cross section surrounded by two radial segments that divide the irradiation surface around an axis that includes the origin. In a cone-divided variable collimator that collimates fast neutron beams from a volume radiation source, the origin is located at the maximum irradiation end of the irradiation part irradiated from the volume radiation source and at the outermost end of the volume radiation source. A fast neutron beam, characterized in that the fast neutron beam is set at a position at least on the opposite side of the volume radiation source and away from the intersection of an extension line connecting the irradiation section and an extension line of the central axis of the irradiation part and the volume radiation source. Variable collimator for treatment.